212 PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 



iied these iiiutual repulsions prevent contact, iind tbe result is a large 

 increase in the number of drops and a dense form of condensation. On 

 the other hand, when the temperature is lowered, surface film repulsion 

 comes into action, contact is prevented, and the drops do not coalesce 

 ou collision, and the result is exactly the same as if they were electrified. 

 In these renuirks no reference has been made to the effect of the dry- 

 ness of the air on the density of the condensation. It seems probable 

 that the relati\e humidity of the air will have a less influence on the 

 density than on the duration of the jet; that is, tlie length of time the 

 drops take to evaporate. 



4. High pressure of the steam. — The fourth cause of the dense form of 

 condensation is high pressure of the steam. If the temperature be below 

 46° the condensation is dense at all i)ressures, but as the temperature 

 rises, the condensation ceases to be dense if the pressure of the steam 

 be low. But if we now raise the pressure, the,iet again becomes dense, 

 and the higher the temperature of the air the higher the pressure must 

 be raised to produce the dense form of condensation. The action of 

 the high pressure in producing the dense condensation is more complex 

 tlian any of the previous causes. It acts, first, by the more rapid move- 

 ments of the jet mixing a larger amount of air with the steam, by which 

 means a greater number of dust nuclei are taken into the jet; and, sec- 

 oiul, a lower temperature is also produced, which probably brings the 

 temperature of the drops low enough for the repulsive action of the 

 films to come into play. But in addition to the eft'ects of a greater 

 amount of air being mixed with the steam, a third action here conies 

 into play. Owing to the violent rush of steam, the condensation takes 

 place more rapidly ; and it has been found that the more rapidly the 

 condensation is effected the gieater is the number of particlea-formed. 

 If tlie condensation takes place slowly, a much less nundjer of nuclei are 

 sufllcient to relieve the super-saturation, as there is time for the move- 

 ments of the water molecules to take place; but if the rate of conden- 

 sation be forced, then the tension of super-saturation compels a great 

 many more dust particles to become centers of condensation. The 

 result of this is, that with two samples of the same mixture of air and 

 steam, if one of them be condensed slowly the clouding is thin, while 

 if the other be condensed (juickly it is thick. This action will come 

 into play in the steam issuing at high pressure, when the steam is rap- 

 idly expanded, cooled, and then mixed with cold air. 



The increased density in-oduced by increase of pressure also takes 

 place somewhat suddenly, thougli not quite so suddenly as when the 

 density is produced by the other causes. The jet first gradually thick- 

 ens as the pressure rises, then a stage is arrived at when it somewhat 

 suddenly becomes dense. When this last stage is arrived at, neither 

 electrification nor the products of combustion cause any increase in 

 the density. The first thickening is i^robably the result of the quick- 

 ening of the condensation and increase in the number of dust nuclei; 



